Proximity detection system and method and collision avoidance system and method using proximity detection

ABSTRACT

The invention relates to a complex proximity safety and warning system. The invention provides a safety system comprising a generator that generates a magnetic field that establishes a boundary, where the generator is capable of receiving radio frequency signals. Also provided is a radio frequency device that sends radio frequency signals, the radio frequency device being capable of sensing the magnetic field and generating a radio frequency response.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.14/479,172, filed Sep. 5, 2014, which is a continuation of U.S.application Ser. No. 13/538,156, filed Jun. 29, 2012, which is acontinuation of U.S. application Ser. No. 11/984,824, filed Nov. 21,2007, which claims the benefit of U.S. Provisional Application No.60/996,034, filed on Oct. 25, 2007, the subject matters of which isincorporated in their entirety by reference herein.

BACKGROUND

The present invention relates generally to safety systems at work sites,and in particular to an interactive magnetic marker field and proximitywarning system. Many methods have been devised to protect people frombeing struck, pinched, crushed or otherwise harmed by vehicles andmobile equipment. Such vehicles and mobile equipment may be used forabove and below ground operations. Examples of the equipment include:road construction equipment such as trucks, road graders, rollers andpavers; surface mining equipment, such as for use with gravel and sandoperations, front end loaders, trucks, dozers, conveyors and otheritems; underground mining equipment such as continuous miners, shuttlecars, conveyors, crushers, load-haul-dump vehicles, man-trips, tractors,and other items. The equipment also includes fork lifts, cranes, andtrucks used at warehouses and shipping ports.

Hundreds of people are killed each year in the US by such equipment.Unfortunately, the systems that have been devised to protect people andproperty in these industrial operations, such as proximity protectionand collision avoidance systems have usually not been very effective. Anew proximity protection system, sometimes referred to as a collisionavoidance system, was developed and successfully demonstrated for use oncontinuous miners, as disclosed in U.S. Patent Application Publication2006/0087443, which is incorporated in its entirety by reference herein.An objective of the '443 publication was to prevent the crushing orpinning of personnel who are remotely controlling a continuous miner,and to protect other personnel assisting in use of the continuousminers. The '443 publication also envisioned to provide protection topersonnel from other types of mobile equipment and machines. The systemof the '443 publication employs a magnetic marker field and an activearchitecture that incorporates two-way communication between the personbeing protected and the machine from which the person is beingprotected. Warnings are given to workers that are dangerously close tothe miner. Warnings are also provided to the operator of the machine.Provisions are made to immobilize the equipment until personnel wereable to reach a safe position.

The '443 publication, however, did not provide an architecture that wasfully adequate for environments where there are many personnel workingin close proximity to multiple equipment in the same workspace. Forexample, it is essential that a signal or response by the system bedirected only to the machine, and/or the machine's operator, that isthreatening a worker's safety. Otherwise, a signal or response from thesystem will result in unnecessary signals or responses to other,unaffected machines and/or workers. Such unnecessary signals andresponses result in unwanted false alarms. False alarms and nuisancealarms have been known for years to be a major reason why many proximityprotection systems and collision avoidance systems have failed when inreal operational environments.

False alarms or nuisance alarms have traditionally been the primaryreason for failure of deployed proximity systems. In real industrialenvironments, proximity systems have experienced many forms of errors aswell as problems related to the shape of the protection zones. Sucherrors and problems are discussed in the NIOSH Report RI 9672, titled“Recommendations for Evaluating & Implementing Proximity Warning Systemson Surface Mining Equipment,” of the Department of Health and HumanServices.

An example of what happens when alarms are sounded without there being areal danger can be explained with respect to standard backup alarmsrequired by law for most industrial vehicles. When the vehicle ormachine begins to back up, a horn is typically sounded. In a workenvironment where there are many vehicles or machines, there typicallyare many horns sounding very frequently. Such horns are soon ignored bythe workers. This is because it is not realistic for each worker to stopto consider every horn sounding within their work area. Even when thereis only one vehicle, if that vehicle is frequently backing up and theworkers frequently hear the horn sound while knowing that their safetyis not being threatened, the workers will soon begin to ignore thewarning horn or alarm. Then, when their safety actually is threatened,the horn provides no protection because the workers would disregard thehorn, believing it to be just another false alarm. Thus, the workershould be warned only when there is a threat to them.

Previous proximity and collision avoidance systems have not beeneffective in reliably warning only of real threats to safety, while alsoavoiding giving alarms when there is no real danger to the worker.Analysis of prior art collision warning systems are discussed in apublication from SAE International, titled “Development and Testing of aTag-based Backup Warning System for Construction Equipment,” No.2007-01-4233. The shortcomings have been found to exist for work siteswhere only one machine and a worker are operating. The shortcomings aremagnified in complex work areas, areas involving many elements. Anotherproximity protection concept has been under development for use at worksites, such as road construction sites, surface mining, loading docks,etc., where multiple machines and vehicles routinely work in closeconjunction with each other, and where many workers work within the areaaround the machines. Based upon tests, a need has been identified forrestricting the defined hazard zones such that workers can approach avehicle at the side or front without producing alarms.

Another challenge for safety systems is that the operators of vehiclesand equipment may frequently dismount and/or leave the equipment thatthey are operating. Existing proximity protection safety systems do notdistinguish between situations when the operator is riding in hisvehicle or machine—situations where the system should not produce awarning or take an action to immobilize the vehicle—and situations wherethe operator dismounts and moves around the vehicle, when fullprotection for the operator is needed.

Given the rapid growth of radio frequency identification (RFID)technologies worldwide, consideration has been given to using RFIDtechnologies and schemes for proximity protection. A major drawback withusing RFID technologies is that this approach depends upon radiofrequency (RF) transmissions at high frequencies in the electromagneticspectrum. Since the maximum range of some types of RF systems is almostunlimited, up to miles, if needed, it might seem to be a good candidate,for that reason, for collision avoidance systems, particularly when thevehicles are traveling at higher speeds. Also, the higher frequenciescan provide much greater bandwidth, which allows implementation of manyspecial functional features. In complex work environments, however,there are many metallic materials and surfaces that reflect the RFsignals, causing the RF signals to travel over multiple paths. If the RFreceivers are used to measure the strength of RF signal, in order todetermine the distance between the vehicle and the person to beprotected, these reflections over multiple paths can cause errors in themeasurements. Radar systems are prone to identify most any objectswithin the defined hazard zones, even though the objects are no threatto safety. GPS signals have also been found to be affected byreflections of nearby equipment, causing a mis-calculation of thedistance between the receivers and the vehicles. As a consequence, areliable marker field (for a safety zone, for example) can not bemaintained with high frequency RF systems. In addition, RF signals donot easily pass through earth formations; as such, personnel may beshielded from the safety system until it is too late to take evasiveactions. Even medium frequency magnetic fields have been found topropagate on cables and pipes, making medium frequency magnetic fieldsless reliable than desired.

In contrast to RF fields, magnetic fields, oscillating at lowfrequencies, are known to be stable and can be effectively used to markoff safety zones or danger zones. Such technologies are discussed inU.S. Pat. Nos. 6,810,353 and 5,939,986 to Schiffbauer, which areincorporated by reference herein. Although the maximum practical rangeof such low frequency magnetic fields may be less than 50 feet in mostapplications, that is more than is needed or desirable for mostequipments. Typical haul trucks would probably be best served with awarning zone in the range of 20-30 feet and a danger zone in the rangeof 10-15 feet. In some applications, such as remotely controlledcontinuous miners, it is necessary for the operator to remain within arange of 10-25 feet much of the time in order to maintain good visualcontact with the machine and the immediate surroundings. In undergroundmines, the magnetic fields pass through earth formations unimpeded sothat a worker that is around a corner, not in line of sight, orotherwise obstructed, will still be visible to the marker field. Thesemagnetic fields do not radiate from antennas but simply expand andcontract around the element that produces them, and are well suited formarking boundaries between safe zones and unsafe zones. An attempt hasbeen made to apply identification information (IDs) to magnetic fieldsas part of proximity protection or collision avoidance strategy. Thereare serious limitations to this approach, however, particularly wherethere are numerous elements (machines, workers, etc.) involved at a worksite. At low frequencies bandwidth is limited, thus limiting theprocesses that would typically be employed. If two adjacent machines aretransmitting their IDs at the same time, the low frequency fields mayconflict, causing the amplitude of the composite field to vary, causingerrors in the data set. With low frequency markers fields, the bandwidthavailable is not sufficient to allow rapidly re-sending data sets anduse algorithms to remove the errors. There are numerous possibleinteractions between many elements, the circumstances of which maysometimes be ignored, but may also be critical to safety.

Conflicts between the fields produced by the multiple systems easilyoccur. Workers may find themselves within the magnetic fields of morethan one machine and coordination of the system responses can bedegraded and unreliable. For example, if a Personal Alarm Device, usedto personally warn a worker of a safety threat, is in two hazard zones,of two machines, and one is a greater threat, the Personal Alarm Devicemust be able to determine which is the greatest threat and respondaccordingly. At the same time, the operator of a machine needs to begiven the appropriate alarm for that machine, not the alarm that isappropriate for the second machine. When there are three or fourmachines that are in the same area, working closely together, it iscritical that the workers around each and the operator of each do notreceive confusing indications. If the alarms are confusing, the safetysystem will not be used. What is needed is a proximity system that canreliably accommodate an environment having multiple moving elements.

There is also a need for a way to transmit information from each workerto log events, such as safety-related event, that are experienced, toallow use of the system to track personnel during an emergency, forexample. There is also a need to provide a means to collect data relatedto the location and safety of an individual worker. In addition, thereis a need for workers to be able to provide interactive responses toequipment and/or operators.

Moreover, although low frequency fields are ideal for marking offprotection zones or danger zones because they are very stable, thisstability in field shape is a disadvantage in some cases. For example,there are situations where it is desirable for workers to be close toequipment at one location but not close at another location. An exampleis a truck that is backing up. A worker at the side of the truck is at avery low risk or possibly no risk at all; yet, a worker behind the truckmay be at a very high risk. Magnetic fields that extend far enoughbehind the truck to provide the needed protection, however, will alsoproduce a larger than desired field to the sides of the truck. There isalso a need, therefore, to be able to shape a marker field to excludeareas where workers need to be positioned, and/or areas that present nosafety risk, for example. Moreover, what is needed is a special systemdesign and architecture for a reliable proximity warning or collisionavoidance system that will avert the many hazards that exist in themany, diverse industrial work environments.

SUMMARY

Work environments to which this invention is applicable are verydiverse. Space in this document does not allow describing all theseapplications. One of these environments, a road construction environmentmay be used as a basis for describing important and novel features ofthe invention. Some workers at a road construction site may be, for atime, working around a particular machine and then move to a differentmachine or continue to work when another machine arrives. Some workersmay frequently move through a space between machines. Yet others may beriding upon a vehicle but may also switch from operating one vehicle toanother vehicle. And, some machines may be remotely controlled by theworkers. A robust proximity protection system must consider and be ableto reliably keep workers safe while coping with all such operationalvariables, and do so without significant false alarms or warnings. Theinvention satisfies these requirements.

Complex work environments pose major challenges to a proximityprotection system when it is required to protect all the movingequipment and personnel from collisions. When there are multiple workersin an area, it is essential that the system's response to one workerdoes not interfere with, disrupt, or confuse the system's actions towardother workers. Therefore, it is important that a warning or alarm bedirected only to the worker who requires such an alarm, but not to otherworkers. This way, an operator or associated worker will know that whenthey receive an alarm, the alarm is specifically intended to alert themto take some action, either to avert or evade a hazard.

In one embodiment, the invention provides a safety system comprising agenerator that generates a magnetic field that establishes a boundary,where the generator is capable of receiving radio frequency signals.Also provided is a radio frequency device that sends radio frequencysignals, the radio frequency device being capable of sensing themagnetic field and generating a radio frequency response. In the safetysystem, the generator generates the magnetic field for a firstpredefined time period, and thereafter senses for a radio frequencyresponse signal from the radio frequency device within a secondpredefined time period.

In another embodiment, the invention provides a safety system forgenerating a shaped safety boundary. The safety system comprises a firstgenerator that generates a first magnetic field having a first boundary,and a second generator that generates a second magnetic field having asecond boundary. According to the invention, overlaying the first andsecond boundaries produces a different boundary, and the first andsecond magnetic fields are generated in sequence.

In yet another embodiment, the invention provides a magnetic fieldgenerator for a safety system, comprising a magnetic field generatorcontroller for producing a timed pulse of oscillations, and an amplifierfor amplifying the timed pulse of oscillations. The generator also has awinding and a ferrite, wherein the timed pulse of amplified oscillationsproduces a current through the winding and ferrite, thereby creating amagnetic field having a strength. According to the invention, thestrength of the magnetic field is variable by adjusting the width of thetimed pulse.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other advantages and features of the invention will bemore clearly understood from the following detailed description which isprovided in connection with the accompanying drawings.

FIG. 1 shows an exemplary worksite having a safety system in accordancewith an embodiment of the invention.

FIG. 2 show schematics of a marker field generator and a personal alarmdevice in accordance with an embodiment of the invention.

FIGS. 3A-3D show a vehicle having a safety system adapted for shaping amarker filed in accordance with an embodiment of the invention.

FIG. 4 shows a shaped marker field of the safety system illustrated inFIGS. 3A-3D.

FIG. 5 shows a schematic of a marker field generator in accordance withan embodiment of the invention.

FIGS. 6 and 7 show timing diagrams of communications between safetysystem components in accordance with an embodiment of the invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

The invention is particularly applicable to work sites that requirepersonnel to be in close proximity to various hazardous elements, suchas machines, mobile equipment, remotely controlled machines, andoperated vehicles. Such work environments may include locations that areinherently dangerous and should be avoided or entered only with greatcaution. Examples of such work environments are surface mining,underground mining, sand and gravel operations, road construction,warehouses, shipping docks, coke plants, etc. Workers are sometimesstruck, pinched, crushed or otherwise harmed while performing their jobsin such environments. Collisions between the various elements at thework sites need to be avoided also to avert property damage.

The invention overcomes the limitations in the prior art by use of anovel method, and apparatus for practicing the method, for generatingand coordinating magnetic marker fields, and for responding to themagnetic marker fields via RF transmissions. Special functions are alsoprovided by the safety system of the invention, functions that make thesafety system more reliable, more user friendly, more versatile and ableto utilize the inherent system capability to provide other benefits.

As disclosed herein, various protection zones can be shaped to fitspecific equipment or work area configurations. Thus, it is possible toproduce safety zones having desired shapes. In addition, an importantaspect of the invention is a method and an apparatus operated to pulsemagnetic marker fields, each pulse being composed of low frequencyoscillations. The pulse is referred to herein as a PING. The apparatusis also operated to pulse RF transmissions, referred to herein as anECHO, in response to the PING. A typical pulse of the magnetic field(PING) would be to produce 73 oscillations of the field, over a periodof 1 millisecond, for example. Typical pulse of the RF response (ECHO)would be to produce 229,000 cycles over a period of 250 microseconds,for example.

The invention is robust, providing a very reliable warning system. Inaddition to the necessity of providing alarms or taking action whenthere is danger, the safety system of the invention does not producefalse alarms when there is no danger. Most previous systems have madelittle or no progress in providing meaningful protection to multiplepeople from multiple machines in the same confined general area. ThePING/ECHO system described herein reliably provides warnings with almostno errors in real industrial environments, involving complex equipmentand personnel arrangements.

Referring now to FIG. 1, there is illustrated a simplified example of awork site where an embodiment of the invention is implemented. FIG. 1shows two vehicles 20, 40, and three Marker Field Generators (MFGs) 22,44, 46. FIG. 1 also shows three personal alarm devices (PADs) 30, 32,34, that are carried by workers A, B, C. FIG. 1 also shows worker D,carrying PAD 36, who is a operating or driving the vehicle 20.

The MFGs and PADs are essential elements of a PING/ECHO system and aredescribed in greater detail with reference to FIG. 2. An MFG 80 is shownhaving a microcontroller 82, which produces low frequency oscillationsthat are amplified by the amplifier 84. The low frequency oscillationsproduce a low frequency oscillating current through a winding 86 aroundthe ferrite 90. A capacitor assembly 88 is connected in series with thewinding 86 to produce the oscillating magnetic field. The values ofcapacitor(s) in the capacitor assembly 88 are selected to produce aseries resonant circuit at the operating frequency of the PING/ECHOsystem. A typical circuit would include an inductance value ofapproximately 300 microHenry with a capacitance of approximately 163microfarad, for example. The invention's use of a series resonantcircuit produces a higher current through the winding 86 with lessdriving voltage.

A magnetic marker field 92 is then produced around the magnetic markerfield generator 80. The magnetic marker field 92 expands and collapsesat the selected operating frequency. An operation frequency ofapproximately 73 kHz has been found to be a suitable choice, forexample. Having knowledge of the teachings disclosed herein, personsknowledgeable in magnetics, tuned circuits and the intended applicationscan select the operating frequency and the circuit elements to fit thespecific applications. For example, the operating frequency might bereduced to 25 kHz if the L/C ratio and the voltage used to drive the LCcircuit do not result in a rise time greater than desired for the systemresponse that is required. Higher frequencies above about 100 kHz shouldbe avoided in underground mining since they tend to propagate on cablesand pipes, or via other metallic objects.

Also shown in FIG. 2 is personal alarm device (PAD) 60. The PAD 60 hasthree orthogonal coils 62 that sense the marker field 92, and in turnpass the sensed signal/information into a detection circuit 64, viafilters 66 and amplifier 68. A microcontroller 70 measures the strengthof the signals, and has a battery 71. The microcontroller 70 turns onthe RF transmitter 72 to send, through antenna 74, a return signal 76(an ECHO) to the receiver antenna 94 in the generator 80. The ECHOsignal 76 then passes to the MFG receiver 96, which registers thepresence of an ECHO and sends an appropriate response/information to themicrocontroller 82.

In use, the MFG microcontroller 82 determines whether the ECHO 76 hasbeen received in response to the PING 92 sent by the MFG 80. The MFGmicrocontroller 82 also determines the safety zone in which theECHO-emitting PAD is located. Alarm signals are sent to an alarm system98 located near the vehicle operator and/or on the vehicle. The alarmsignals are audible and/or visible to the affected worker(s). Thehousing 81 of the MFG 80 shown for this configuration may to be anexplosion-proof type, so that it may be used in underground coal miningor other applications where there may be an explosive environment. Otherdetails regarding MFGs and PADs, including for example how housings maybe constructed, are disclosed in the '443 publication, and incorporatedby reference herein.

Referring again to the work site shown in FIG. 1, there are numerouspossible cooperative and potentially destructive relationships betweensystem elements, even for this simple example. With more elements, thenumber of possible relationships increases rapidly. A real operationalenvironment often includes many MFGs and many more PADs. For example,with four vehicles or machines, four operators, and four other workers,there are hundreds of relationships and possible conflicts between thevarious PADs and multiple safety zones generated by the MFGs. As anexample, if all the marker fields are pulsed continuously, then at anypoint the composite field between the marker fields is the vector sum ofthe fields. If a marker field also includes any IDs or similar data, itmay be overridden by other marker fields or cause conflicts with othermarker fields, producing errors. The PADs must be able to measure any ofthe marker fields in which they are located, to measure them, to knowwhich system (or MFG) produced the marker field, and respond only to themarker field that should be responded to.

For example, consider a situation where a shuttle car is approaching acontinuous miner to load coal. The shuttle car must approach acontinuous miner until it makes contact with the miner, so that there isa significant overlap between marker fields generated by the twomachines. There can be conflicts between the marker fields, which inturn is problematic for PADs being used in those areas. If the fieldsinterfere or the PADs are unable to respond to both machines, theworkers will not be fully protected.

It is sometimes very important that the safety system's warningscorrespond appropriately to at least two levels of threat, while givingpriority to the most serious threat. For example, in FIG. 1 two zonelevels are defined by the magnetic marker fields. A Warning Zone 28, 58is an area that should be avoided because danger is approaching. ADanger Zone 26, 56, is an area where a worker would be in imminentdanger and should move immediately to a safer location, or should use anemergency stop feature, on his PAD for example, to stop the machine. Inaddition, Monitor Zones 24, 54 can be used to alert a worker that theyare in the vicinity of a machine that is equipped with proximityprotection, even though they are not in danger and may not receive analarm. Other additional zones can be defined to allow other functions tobe performed, such as authorizing PADs to transmit data for recordinginformation from the system or environment that the PAD is sensing.

Some illustrations of situations that the safety systems must be able tohandle are as follows. With reference to FIG. 1, worker A carrying PAD30 and a worker B carrying PAD 32 are initially at the rear of vehicle20, as shown. Worker B carrying PAD 32 then moves out of the Danger Zone26 and into the Monitor Zones 24, 54, of vehicles 20, 40. Meanwhile,worker C, carrying PAD 34, leaves the cab of his vehicle 40 and movesfrom Danger Zone 56 to Danger Zone 26 at the rear of the first vehicle20, to join the other two workers A, B. To accomplish these changes inposition, workers B, C, have passed through multiple safety zones(marker fields) of two vehicles 20, 40, Worker C carrying PAD 34 passedthrough at least four zones, and passed though zones of both vehicles20, 40, at the same time.

Not only must the magnetic marker fields of the two trucks 20, 40 notconflict, but also the PADs 30, 32, 34 must not conflict with each otheror the MFGs. It is important that the systems be able to recognize thatthere are workers in their Danger Zones 26, 56, and if not, whetherthere are workers in their Warning Zones 28, 58. According to theinvention, the PADs 30, 32, 34 warn the workers and the system warns thedrivers of the trucks, without one safety system component masking orconfusing another. The system of the invention, which includes, forexample, the PING/ECHO technology as discussed in detail below, allowsall these interactions to occur rapidly and without conflict.

It has been demonstrated in the '443 publication that an oscillatingmagnetic field, operating at low frequencies provides a dimensionallystable marker field. It is also known that such low frequencyoscillating fields will penetrate most materials, including earthformations; yet, do not radiate energy so that there are no multi-pathreflections. The lack of radiation is due to the size of the generatorelements being very small compared to the wavelength of the field, beinggreater than 4,000 meters for a 73 kHz oscillation. Another benefit fromusing non-radiating magnetic marker fields is that the strength of themagnetic field varies inversely with the cube of the distance from thefield generator, as compared to RF emissions from antennas which reduceinversely with the square of the distance. This cubic relationshipreduces the amount of conflict with other nearby marker fields, ascompared with radiated RF waves, and also allows greater accuracy whendetermining distances from the sensors to the generators.

The invention provides a reliable and accurate method for allowing manyactive elements to be compatible and to cooperate with one another. Theapproach, in one embodiment, is to generate short pulses of magneticfields in a controlled, semi-randomized manner, and, to obtain responsesfrom PADs or TAGs, carried by workers, that sense those pulses. Bygenerating short pulses in a semi-randomized manner, at the time ofgenerating a PING, a timing window is set and the specific point withinthat timing window when the oscillation is to be produced is chosen byuse of a random number generator. Also, if a burst of PINGs aretransmitted, in response to which a burst of ECHOs may be produced, thetime spacing between the PINGs may be a small timed window, which may beonly a few milliseconds in duration, for example. The system is arrangedso that multiple ECHOs from multiple PADs, and reflections of the ECHOs,will not impair proper operation. In addition, flexibility is providedwithin the system to allow important special features to be provided.For example, instead of generating a single PING, several PING can beproduced sequentially by multiple generators, and the PAD can beprogrammed to wait until all the PINGs have been received before makinglogical decisions on the action to be taken.

There are various special situations that must be considered. Forexample, if a worker is positioned between two vehicles, a vehicleposing no threat to the worker must not mask the closer, or moredangerous vehicle. It is also critical that the operator know if anyworker is potentially in danger, regardless of how many workers are inthe work area. And, if automatic shutdown or slowdown features are beingused by machines, the system must also be able to determine if anysituations have reached such a level of potential danger. To accomplishthese and other important functions, the components of the safety systemprovide logical functions, and communicate quickly and reliably. This isaccomplished by using the combination of the short duration PINGs andshort duration ECHOs, implementing narrow time duration windows forreceiving PINGs and sending ECHOs, random time generation, and logicaldecision-making on both ends of the information exchange.

A PING/ECHO system provides a sound basis for the required systemcapabilities. PINGs are short in duration and are sent multiple timesper second, in a semi-random manner so that there is time for PADs toreceive PINGs from many sources. If the PAD is in a Warning or DangerZone, it responds with an ECHO via an RF transmission of a particularduration at a particular time, information which indicates the zone thatthe PAD is in. Each system which produces PINGs also has a current alarmstate representing the safety system's knowledge of workers in theprotected zones. If a PING/ECHO sequence has occurred and indicated tothe system an alarm level higher than the current alarm state, thesystem may produce a rapid burst of two or three or more PING/ECHOexchanges to verify that the initial PING/ECHO exchange was correctbefore entering a higher alarm state. The burst mechanism is designed toavoid false or nuisance alarms. If a PING/ECHO sequence fails to confirma state, the process can be repeated multiple times per second.

Since the PINGs are sent out at semi-random time intervals, it ispossible that two MFGs can produce PINGS which overlap in time. If a PADis in the proximity of both of these MFGs, it may misread the fieldstrength and may produce an erroneous ECHO in response. The burstconfirmation sequence, in which multiple PINGs are sent as a group, butare separated by a short fixed or random period of time, is designed togreatly reduce the statistical likelihood of false alarm signals. ThePINGs and the burst confirmation sequence are randomized to such anextent that it is very unlikely that the system will erroneously enteran alarm state even if several systems are operating near to each other.For example, once the first PING/ECHO exchange has been completed, threepulses can be sent with the condition that the space between the pulsesbe selected randomly from a set of numbers ranging between 250microseconds and 5 milliseconds.

An important aspect of the PING/ECHO system is that all the PADs (orTAGs) in a given work complex can be made to respond in exactly the sameway or some can be made to respond in different ways. Making all PADs orsome groups of PADs in a work area to respond in an identical manneroffers advantages. For example, they can each be programmed so that thesignals from multiple PADs that happen to be in the same zone at thesame time will reinforce each other. With this approach, a workercarrying a PAD can move from one work site to another, within thecomplex, and be sure of having the same level of protection. But, wherespecial circumstances require, a PAD can be programmed uniquely and thesystem on a machine can be programmed uniquely. For example, if a truckis being protected from backing over a spoil and falling into a deeppit, a PAD can be attached to a pole that is position to be in thedanger zone when the truck backs up to the desired dump point. It can beset to sense the field produced by an inductor at the rear of the truck.The inductors at the rear of the truck would typically define a hazardzone. The sensitivity of this PAD may need to be set in a differentrange that other PADs worn by workers, though not necessarily. Thecircumstances and operation scenarios would need to be considered. But,the PAD could also be programmed to respond only to the generator at thefront of the truck, which would have a field extending to near the lastaxle of the truck, for a typical PAD setting. The PAD would know whichgenerator was at the front of the truck because of the known sequence inwhich the fields would be produced by the three generators (a typicalconfiguration). However, this must be done very carefully so that use ofthe standard configured PADs will continue to provide the sameprotection.

In some work areas, workers must be close to the side of vehicles ormachines, but should not be behind those vehicles or machines, at leastwhen they are backing up. It is helpful in those cases to narrow themarker field so that the workers can be closer to the side of thevehicles without initiating an alarm on their PAD or alarm the operator.Since one important characteristic of the low frequency magnetic markerfields is that their shape tends to be very predictable and constant,the fields cannot, from a practical standpoint, be shaped to be madenarrow at the side of vehicles or machines. The invention discloses useof multiple marker field generators to modify the shape of the markerfields. One way to modify the shape of a magnetic marker field is byreversing the phase of one or more of multiple generators. More controlover the field shape can be accomplished by considering the fact thataccording to the invention the marker fields may not exist concurrently.By requiring a PAD to be in the same zone of two generators, the shapeof that zone is reduced to the area of overlap by the fields because theareas of overlap of the two generators is smaller, located between thegenerators. If inductors that are generating the fields are, forexample, at the rear corners of a truck, then the overlapping fieldswill be centered on the rear of the truck and their width with bedetermined by the strength of the fields and the sensitivity to whichthe PADs have been programmed.

Further, the requirement can be placed on the PAD, by use of thesoftware in the microcontroller, to exclude areas of overlap from thezone. By combining the PING/ECHO method with logical capabilities in themicrocontrollers, significant shaping of the protection zones isaccomplished. For example, the zone for a truck backing up can beginnear the rear of the truck and extend behind the back of the truck withan arrowhead shape. This produces an ideal zone of protection. Personnelforward and rear of the tailgate would not be in an alarm zone and thedriver would not be in an alarm zone.

Multiple marker fields can occupy the same space by limiting each one toshort periods of time duration, producing a pulse or PING that, forexample, may only be about 1 millisecond in duration, which is about 73cycles of the 73 kHz oscillation. Since there is the possibility thattwo generators in close proximity might happen to be synchronized, sothat the PINGs from the generators occur at or nearly the same time, thetime durations between the PINGs are randomly chosen within a timewindow (FIGS. 6 and 7). This way, if the generators happen to besynchronized, the probability of a conflict between responsive ECHOs isgreatly reduced.

A known problem is that ECHOs in the form of RF transmissions from otherPADs, in response to other PINGs, could, in error, be accepted as ECHOsby the MFG 80 (FIG. 2). To address this problem, the receiver 96 in theMFG 80 looks for the return ECHO to appear during a specific timewindow, for example a time window of one or two milliseconds followingthe sending of the PING. Moreover, a predetermined time delay may beimplemented after sending of the PING and before the specific timewindow begins. This way, in order for a stray pulse from some othersource to be erroneously accepted as a responsive ECHO, it must occurwithin the precisely defined window.

But, there is still a possibility that an error could occur, even thoughthe possibility is small. To make sure that the RF signal received is avalid ECHO, the system of the invention can be operated to send a seriesof PINGs in a burst, and then the corresponding ECHOs are received andanalyzed. This burst of PINGs is performed quickly before the next PINGwindow is available from any other MFG that might have sent a PING atthe same time. In the unlikely case where a PAD is in the same zone oftwo MFG systems on two vehicles or machines, and they also happen tohave been synchronized at the moment that the PAD entered the zones ofboth MFG systems, that PING/ECHO burst sequence would be rejected.However, the whole process will begin again within the next PING window,when the next randomly generated PING will be sent, typically between0.050 and 0.250 seconds later. The extremely small probability of thisentire sequence coincidentally happening again is so small that it canbe ignored. In order to verify that the ECHO is valid, the abovesequence can be repeated several times, very quickly. If all responsiveECHOs to the repeated sequences of PINGs are not received as expected,the microcontroller 82 assumes that the initial ECHO was invalid andtakes no further action unless subsequent PINGs result in a valid ECHO.But, if the ECHO is confirmed to be valid, the microcontroller 82 willtake the action it has been programmed to take.

An alternate strategy to using a burst of PINGs to ensure that an ECHOis valid is to produce PINGs on a random basis, within selected timeperiods, and to require that a sequence of PINGs and ECHOs be receivedto establish that a valid reading and response has been accomplished.Also, the timing windows established for receiving the ECHOs can bevariable within a string of PING/ECHO exchanges to further reduce theprobability of noise from other sources being interpreted as PINGs orECHOs. The length of the PINGs and ECHOs and the receiving windows mayalso be varied in timing and/or in duration for responses representingdiffering levels of danger. Once the PING/ECHO system has beenarchitected according to the invention so that the PING is produced bythe magnetic marker field and the ECHOs are returned by RF link variouscombinations of timings and durations of transmissions and space betweentransmissions may be employed.

It can be appreciated, therefore, that any generator can produce itsmarker field without interfering with another field. It also followsthat the ECHO can reliably indicate that a PAD is either in the WarningZone, Danger Zone, or other defined zones, without a conflict. Themicrocontroller 82 in the MFG 80 can then reliably give priority to anECHO from a PAD in the Danger Zone over the priority given to a PAD inthe Warning Zone, for example.

Some of the versatility of the PING/ECHO concept is demonstrated by theway that the protection zones or danger zones can be shaped. A magneticmarker field generated by use of a ferrite has a shape with across-section that is approximately that of an ellipse. As explainedabove, the marker field is stable and reliable, and maintains its shape.If multiple generators are used, however, the shape of the resultingmarker field can be modified by the techniques disclosed herein. Twogenerators can be used to produce overlapping marker fields, and betimed so as not to occur at the same instant in time. The PADs to beused in such work sites can then be programmed to only alarm when thePAD is within the zone of both generators, i.e., only when the PADsenses PINGs from both or multiple generators. If needed, it is possibleto have three or more generators on large equipment, to produce evenmore restricted fields. In addition, the overlapping fields can beadjusted to have different strengths in order to further alter the shapeof the zones. According to the invention, adjustment of the strength ofthe fields produced by each MFG can be accomplished by adjusting thewidth of the pulses that drive the current through the generator. It isalso possible to program the microcontroller in the PADs to havedifferent sensitivity to the MFG fields which will adjust the size ofthe fields, thereby adjusting the size and shape of the hazardous zoneson a vehicle or machine. It is also possible for the microcontroller tooutput the signal to the generators such that the fields are opposingrather than reinforcing each other in order to produce special fieldconfigurations. Three or more generators can be used to further restrictor modify the protection zones or danger zones.

An example of the effectiveness of the zone shaping feature is shown inFIGS. 3A-3D and 4, in which the effect of combining marker fields ofthree generators is shown in sequence. The FIGS. have been preparedbased on actual test data and tests conducted by the inventors. FIG. 3Ashows a vehicle 120 having a front 122 and a rear 124. In FIG. 3A, aWarning Zone 102 and a Danger Zone 104 are pulsed or otherwise generatedby MFG 100. The MFG 100 is located at the left rear of the vehicle 120.In FIG. 3B, MFG 140 is shown, being located at the right rear 124 of thevehicle 120. MFG 140 creates a Warning Zone 142 and Danger Zone 144.Next, FIG. 3C shows the areas of overlap of the marker fields producedby generators 100, 140 (as shown in FIGS. 3A and 3B). In an embodiment,it is these areas of overlap that are to be avoided by the workers. FIG.3D shows a Warning Zone 172, a Danger Zone 174, and a Safe Zone 134 thathave been constructed by overlapping the marker fields. The Warning Zone172 and the Danger Zone 174 are created by requiring a PAD to determineif it is within the Warning Zones 102, 142 of both MFGs 100, 140, on therear of the vehicle 120. Similarly, the PAD will determine if it iswithin both Danger Zones 104, 144 of those generators. The Safe Zone 134is created by adjusting the strength of the field produced by the MFG130 on the front of the vehicle 120 such that the Warning Zone reachesto approximately the location of the third axle of the truck, forexample.

A completed shaped zone of the safety system is shown in FIG. 4. FIG. 4shows the complete shaped zone, adding the Safe Zone 134 produced by thethird generator 130. In the shaped zone, the Safety Zone 134 is excludedfrom the overlapping Danger and Warning Zones created by the MFGs 100,140. One technique for excluding the Safety Zone 134 is by having thePAD recognize the sequence in which the maker fields are pulsed (forexample, the Safety Zone 134 would be the third PING sensed by the PAD)and recognizing the particular strength of the Safety Zone 134 createdby MFG 130. FIG. 4 shows three workers 162, 164, 166, and a driver 160around the vehicle 120. Two magnetic marker field generators 100, 140,shown mounted (and shown electrically connected in FIG. 5) have beenplaced near the rear 124 of the vehicle 120. The fields, indicated bythe zone boundaries 182, 184 from these generators roughly approximatethe shape of an ellipse, but are combined to define the Warning Zone 172and Danger Zone 174. These zones 172, 174 should be avoided, except thata worker in the position of worker 166 really is in no danger and mayhave a need to be in that position. A novel aspect of this invention isto add a field that represents a Safe Zone 134 that can be used tosupersede the effects of the overlapping hazard zones, but only in anarea known to be safe. If a PAD is within the Warning Zone of aparticular field, produced by a particular inductor, that knowledge isused to ignore being within the zones of other generators. Therefore, ifthe intersection of two danger zones produces a field that is largerthan desired in some direction, the use of the third generator allowslimiting the size of the field in the direction of the third generator.

As explained earlier, the overall size of the field produced by eachgenerator is determined by the power put into the generators, and by thesensitivity of the PADs that detect the fields. The input power and thePAD sensitivity can be adjusted by changes in the software, for example.Changes to these parameters, made by the software, could be initiated byuse of switches on the PADs or by use of switches that signal amicrocontroller to make such adjustments. While the two generators 100,140 at the rear of the truck define the Warning Zone 172 and the DangerZone 174, a third generator 130 defines a Safe Zone 134. The thirdgenerator 130 is shown located forward of the other generators 100, 140,near the front 122 of the truck 120. It could also be positioned atother forward locations. The marker field produced by this third MFG 130defines the Safe Zone 134. When in this zone, there will be no alarmsfrom PADs, such as for worker 166 or to the operator 160. Having definedthe zone to be avoided at the rear of the truck and having defined aSafe Zone forward of the tailgate, the resulting shaped zone to beavoided is made ideal.

Now, explanation on how the pulsing from the PING/ECHO system functionsin combination with the pulsing from PADs. A block diagram shown in FIG.5 depicts a practical arrangement of system components to produce shapedhazardous zones and safety zones, as described above. Microcontroller202 produces a 73 kHz oscillating waveform, typically for a period ofone millisecond. Although the frequency of the oscillations and theduration of the pulse of oscillations can be programmed, they must becontrolled very precisely. The microcontroller 202 decides whichinductor 204, 206, 208, is required in the pulse sequence, and sends thesignal to the amplifier 210, 212, 214, associated with that inductor.These signals are then amplified and the resulting current is sentthrough the series combination of the capacitor assembly 216, 218, 220and the inductor 204, 206, 208.

The capacitance assemblies 216, 218, 220 may include adjustments toallow tuning of the circuit to resonance in order to maximize the outputof the field. Or, they may be adjusted to de-tune the circuit to reducethe amount of current, and, thus, the size of the marker field. Asexplained earlier, the average oscillating current through the tunedcircuit can be adjusted by programming the width of the waveform sent tothe amplifier. With reference to FIG. 4, a PAD worn by a worker 162located in the region 172 will measure the strength of the fieldsproduced by both MFGs 100, 140 and determine that it is measuring afield strength from both MFGs 100, 140, that indicate it is in theWarning Zones of both. Pulses from the MFGs are produced by themicrocontroller 202 in the MFG Control Assembly 200, amplified by theamplifiers 210, 212, 214, and then forced out around the inductors 204,206, 208. These pulses are produced in sequence by the three inductors,so that only one inductor is producing a marker field at any point intime. These field pulses are the PINGs, as discussed herein.

When the PAD worn by worker 162, shown to be in the cross-hatched area172 determines that it is in the Warning Zone of both generators, it isprepared to alarm the worker who is carrying the PAD and to send awarning to the operator 160 in the vehicle. The PAD, however, alsochecks to determine if it is in the safe zone. If not, it sounds analarm and may also illuminate an LED, and send a signal to the operator160 to sound an audible alarm 222 and to illuminate an LED(s) 224, 226,to alert the operator that a PAD is in the Warning Zone. If it were inthe Warning Zone of the third inductor, such as at the location of PAD166, it would indicate that the worker is too far forward toward theback of the truck to be in danger of being backed over, meaning that itis in a Safe Zone 134, and would therefore not set an alarm. However,since the PAD is to the rear of the truck, it will not determine that itis in the field of the third inductor and, therefore, it will sound analarm to the worker, both audibly and visually, and alarm the truckdriver 160, both audibly and visually.

Now, consider the third PAD worn by worker 164. For the configurationshown in this illustration, there is an area where the Danger Zones ofthe two generators at the rear overlap, shown as double cross-hatched,and PAD 164 is located in this zone. PAD will sound a Danger Alarm andinitiate an alarm in the cab of the vehicle, both audibly and visually.When PAD 164 looks for the PING from the third generator 130, it willnot be seen because the safety zone field has been adjusted to end a fewfeet forward of the back of the truck 124. Therefore, it will act uponbeing in the Danger Zone 174, alarming the worker 164 and alarming thedriver 160 in the cab.

An exemplary diagram of communications between MFGs and PADs is shown inFIG. 6, where lines M1, M2 and M3 depict the output (pulses) of MFG1,MFG2 and MFG3, respectively, as each MFG sends a PING 190. Lines P1, P2,P3 and P4 illustrate an output of PAD 1, PAD2, PAD4 and PAD4,respectively, the output being VHF radio frequency ECHOs 192. In thisexample, all three MFG's can receive the signals from all four PAD's,and vice versa. MFG1 looks for the presence of an ECHO signal during thepredetermined time windows labeled W1A and W1B. MFG2 looks for thepresence of an ECHO signal during the time windows labeled W2A and W2B.And, MFG3 looks for the presence of an ECHO signal during the timewindows labeled W3A and W3B.

In the illustrated example, PAD1 senses the PING 190 from MFG1, measuresthe signal strength and determines that it is in a danger zone, andresponds by emitting a pulse on the VHF channel, after a predeterminedtime delay. An exemplary predetermined delay is approximately 1millisecond, but can be up to approximately up to 10 millisecondsdepending upon other considerations. PAD2 senses the PING 190 from MFG1,measures the signal strength and determines that it too is in thewarning zone, and responds by emitting an ECHO on the VHF channel, aftera predetermined time delay, which may be different that thepredetermined time delay for PAD1. PAD3 senses the PING 190 from MFG3,measures the signal strength and determines that it is in the dangerzone, and responds by emitting a pulse on the VFH channel, after apredetermined time delay. PAD4 senses the Ping from MFG3, measures thesignal strength and determines that it is in the danger zone, andresponds by emitting a pulse on the VFH channel, after a predeterminedtime delay.

Since MFG1 detects an ECHO on the VHF channel during time window W1A, itinterprets that as an indication that a PAD1 is present in its markerfield within a danger zone. MFG1 also senses a VHF signal in time windowW1B and determines that PAD2 is also present within a warning zone. MFG2does not sense any ECHOs on the VHF channel during any of its timewindows W2A and W2B, and concludes that no PAD is in either of itsdanger/warning zones. MFG3 detects a signal during time window W3A, andconcludes that there must be at least one PAD in its danger zone. Sinceit is possible for an unrelated pulse to occur in these time windows,the MFG's do not set off an alarm condition until they receive an ECHOin the same time window following at least two successive PINGs, forexample.

Referring now to FIG. 7, details are provided regarding how pulsing ofPINGs and ECHOs can be timed to make the PING/ECHO exchanges occurwithout conflict. The left half 300 of FIG. 7 represents a PING/ECHOevent for a PAD that enters or is within a Warning Zone. The line 302 isa timeline showing PING pulses 304 produced by an MFG. The line 306 is atimeline of ECHO pulses 308 produced by a PAD. The ECHO pulses 308,according to the invention, occur after a predetermined delay 324 thatmay be from less than one millisecond to a few milliseconds. Element 310is a detail of the ECHO event, and shows the ECHO 308 within the ECHOevent 320. The ECHO event 320 is comprised of time periods 330, 332, 334and 336. The ECHO event 320 represents the time duration window presetin the MFG to receive the ECHO. ECHO bar 308, inside the ECHO event 320,represents a short pulse from the PAD that must occur in the first timeportion 330 of the time window 320, for example.

The right side 320 of FIG. 7 represents a PING/ECHO exchange for a PADthat enters or is within a Danger Zone. Note that the ECHO 328 in thisexample is required to be long enough to appear in all time portions330, 332, 334, 336, of the time window 320. Also shown are twoadditional PINGs 304, and two additional ECHOs 328. The three PINGscomprise a burst, in which the PINGs may be separated by 1-10milliseconds. In the illustrated embodiment, the MFG and the PAD set analarm after all three ECHOs have been received within a time window.With PINGs being very short, multiple PINGs can be generated each secondby a MFG, assuring that a PING/ECHO exchange can be made withoutconflict. A statistical calculation can be made to indicate the highprobability of successful PING/ECHO exchange without conflict from anearby system. A PING may be sent, for example, approximately once each0.125 seconds, on the average, within a window of 0.050 and 0.250seconds.

The simple chance of a successful exchange without a conflict betweenthe PINGs from two systems is 124/125 during the first window. Theprobability of not being able to make a successful exchange in a fullsecond is equal to ( 1/125)⁸, roughly 6×10⁻¹⁶. Whenever multiplegenerators, many PADs, longer pulses for shaping, and the possibleconflicts between ECHOs are considered, the probability of a successfulexchange in even a half second is very large, remains so large as to notto be of any practical concern. Similarly, the probability of a falsePING/ECHO happening, considering the requirement of three complete,randomized PING/ECHO exchanges, is very small. Tests have confirmed veryhigh reliability. It should be noted that the problems mentioned earlierconcerning multi-pathing, reflections, interference, etc do not affectthe ECHO since the requirement is only to receive a pulse of specifiedduration at a specified point time.

Note that in FIG. 4 that the truck driver 160 is in the Safe Zone 134 sothat his PAD will not set an alarm and he is free to leave the truckwithout entering the Warning Zone 172 or Danger Zone 174. Other workersmay approach the driver 160 to have a face-to-face conversation withoutsetting off an alarm. Provisions can also be made for use of a reversesignal from the truck system to turn off the protection system exceptwhen the truck is in reverse, as was done in the evaluation described inSAE:2007-01-4233. However, with the Safe Zone 134 being defined the wayit is, the need for a reverse-only protection is reduced. In someinstances, having the system activated only when the vehicle is inreverse can create an unsafe condition since there may not always be alengthy delay between the time that the truck is placed in reverse untilit begins to move backward. Safety reports on fatalities show this to betrue. An example is a case where a vehicle is parked on a slope so thatit can begin moving in reverse before the clutch is engaged

With reference to FIG. 5, potentiometers 228, 230, are provided in theoperator monitor 232, which is typically located in the cab of thevehicle, that are used to reduce the size of the magnetic field if thatis desired. Holes are provided in the housing of the Monitor so that ascrewdriver may be inserted to make the adjustment. A voltage derivedfrom the position of the potentiometers 228, 230 is used as an input tothe microcontroller 202 which makes a reduction, if necessary. Thereduction is accomplished within the control assembly 200 by shorteningthe width of the 73 kHz square wave signals that drive the amplifier210, 212, 214. By shortening these signals, less energy in input to theresonant LC circuit, thus reducing the average current through the LCcircuit and reducing the size of the magnetic field.

Signals are available at the operator monitor 232 to be used in thevehicle's electrical system to slow or stop a vehicle if that isappropriate for the particular vehicle in a particular work environmentwhere this feature is desirable. Additional lights or horns can beinstalled on the rear of the truck to be sounded for a Warning or Dangercondition. The lights can be made to flash at differing rates todifferentiate a warning condition from a danger condition. Similarly, ahorn can be sounded in different ways for the same purpose. The PADusually provides a beeping for a warning with the speed of the beepingincreasing as the PAD is further into the Warning Zone, and then givinga constant beep or a fast triple beep to indicate that it is in theDanger Zone.

The systems of the invention may be installed near a particularlydangerous zone, which could be related to the geology of that particularlocation, so that a person carrying a PAD will be warned before enteringthe zone. The system may also be installed on dangerous equipment andmay be operated so that the dangerous equipment will be stopped whenevera person approaches too closely. The equipment may be stopped followingor commensurate with issuing a warning to the person by audible soundsor lights. These warnings may be issued by the PAD or by the system thatis installed on the equipment, or both. Zone monitoring systems of thistype can may be battery operated, making them portable.

Whenever a PAD enters a defined hazardous zone, the logic in themicrocontroller is instructed to operate in a special way, called beingin an ALARM STATE. Once in this ALARM STATE, it begins to perform otherfunctions that need to be performed once it has been determined that thePAD is within a hazardous zone. The active ALARM STATE directlydetermines, for the protected machine, which (if any) alarms are soundedand/or which other actions are taken such as slowing or stopping themachine. The system also keeps track of other internal stateinformation—such as the history of the ECHOs received in response torecent PINGs. The system enters a higher ALARM STATE only after multiplePING/ECHO sequences have confirmed that a PAD is in a zone where adanger exists. The system enters a lower ALARM STATE only when someperiod of time has elapsed during which it has received no ECHO of thetype which would indicate that it should remain in the elevated ALARMSTATE.

The invention provides other features that work in conjunction with thePING/ECHO proximity protection system. For example, data is transmittedvia an RF link from the PAD on a regular basis to provide informationrelated to the safety of the worker and allows tracking of the workerduring emergency conditions, the latter being most needed in undergroundmining. Also, the system provides a worker with the capability to useswitches on the PAD to make responses to the machine, machine operator,or to signal through other communications channels, particularly duringan emergency. Provisions may also be made to use the RF link in the PADto transmit voice signal, by adding a microphone to the PAD, or totransmit other information available to the worker and regarding theworker's environment, such as dangerous gas concentrations, vital bodyfunctions of the worker such as heart rate, etc. by inputting theinformation to the RF transmitter. Tracking nodes can be placed in thework area to collect such information from PADs and forward thatinformation to other locations. Memory modules are included in MFGs tostore safety related information for routine safety evaluation or foranalysis following an incident. For underground mining applications,this can include sending information to VLF or ULF Through-The-Earthtransmitters during emergencies when communications to the surface havebeen disrupted or through MF transmitters within the mine. The PADs orTAGs used with the PING/ECHO system may be combined with the cap lampassembly that is worn in underground mining so that they can draw powerfrom that source. Other sensing devices can be integrated into thePAD/Cap Lamp Assemblies in order to take advantage of the RFtransmission capability, portable power, and the microcontroller.

The invention can also be used to avert collisions between vehicles whentheir speed is sufficiently slow, less than about 20 feet/sec (approx 6meters/sec), for example. At higher speeds, the size of the fieldrequired to give a timely warning may be greater than the marker fieldthat is produced. The size of the marker field may be limited because todouble the size of the field, the current through the inductor must beincreased by a factor of eight for a given input voltage, which wouldrequire significant equipment modifications. A PAD, or another similardevice, is placed on the vehicle, along with a MFG. A circuit from theMFG to the PAD is used to disable the PAD during PINGs from the MFG. Ifthe PAD senses another vehicle in its Warning Zone or Danger Zone, italarms the operator of the vehicle. Signals from the MFG can be used toslow or stop the vehicle if that is desirable.

Situations arise in which there is a need to warn a driver of a vehiclethat the vehicle should be stopped, even though there are no otherworkers close to the vehicle. For example, many people have been killedand others have been seriously injured when drivers backed trucks to ahigh dump point, but have gone too far and the truck becomes stuck, orworse, falls over the ledge of the dump. A TAG can be positioned at dumppoints to let the driver know when they should back up no further. Sucha TAG can also be located on loading docks so that a truck driver can beinformed when the truck is about to contact the dock, thus preventingdamage to the truck and/or dock. Operators of vehicles may be frequentlygetting onto their vehicles and then getting off again in order toperform various tasks. When on or in their vehicles, the operators maynot be in danger of being harmed by other equipment. But, when theoperator leaves the vehicle, the operator may then be exposed to threatsfrom other vehicles or dangers and needs to be protected. This specialprotection feature can be complemented by use of a switch on the PAD.

If a worker approaches a vehicle that does not currently have an activeoperator, that worker can request that the system designate him as theoperator. This is accomplished by pressing a button on his PAD while inthe Warning Zone for that machine. The PAD carried by that worker willsend the request, along with the ID of that PAD. The MFG will thendesignate that PAD as belonging to the operator and send a special pulseback to the PAD while the button is depressed. That PAD will no longeralarm and the MFG will ignore it as well. The PAD will periodicallytransmit its ID, typically each 10 seconds, as part of a data set. Solong as that PAD remains at or on the vehicle, another worker cannotbecome the operator of that vehicle. If the current operator leaves thevehicle but returns before another operator has been assigned operatorstatus by that vehicle, the PAD will recognize that the PAD still hasoperator status and will not respond.

There can only be one operator per vehicle, however. If the operatorleaves the vehicle such that the data set is not received within thetime period planned for the periodic data sets, to show that theoperator is still within the marker field, then the operator status ofthat PAD will be placed in suspension. If another worker requestsoperator status for that vehicle while the earlier operator hassuspended status, the new worker will be accepted as the operator andthe earlier operator will lose his operator status. If power to the MFGis briefly interrupted, the PAD may alarm until the next data set issent or until the operator presses the switch again. If the operatorturns off power to the MFG and dismounts, the process will start at thebeginning when another worker arrives or that operator returns andrequests operator status. Whenever an operator with a PAD leaves themarker field of the vehicle for which it has operator status, the PADwill respond to other marker fields in a normal fashion. From theoperator point of view, this process will be simple, essentiallytransparent. When the operator turns on the power to his vehicle, hisPAD will begin to sound. The operator presses the switch on the PAD andthe warnings will no longer be given and the PAD will not respond to anyPINGs from marker fields.

The above description and drawings are only illustrative of preferredembodiments of the present inventions, and are not intended to limit thepresent inventions thereto. Any subject matter or modification thereofwhich comes within the spirit and scope of the following claims is to beconsidered part of the present inventions.

What is claimed as new and desired to be protected by Letters Patent ofthe United States is:
 1. A system, comprising: a vehicle; a first fieldgenerator that generates a first magnetic field; a second fieldgenerator that generates a second magnetic field; and an alarm devicethat sends response signals, the alarm device being configured todetermine a strength of the first and the second magnetic fields,respectively, and to generate an alarm if the strength of the firstmagnetic field is above a first predetermined threshold and the strengthof the second magnetic field is not above a second predeterminedthreshold.
 2. The system of claim 1, wherein the alarm device is furtherconfigured to not generate an alarm if the strength of the firstmagnetic field is above the first predetermined threshold and thestrength of the second magnetic field is above the second predeterminedthreshold.
 3. The system of claim 1, wherein the alarm is an audiblealarm.
 4. The system of claim 1, wherein the alarm is a wirelessresponse signal.
 5. The system of claim 1, wherein the first and/orsecond field generator is positioned at the vehicle.
 6. The system ofclaim 5, wherein the alarm device is positioned at a dump point.
 7. Thesystem of claim 5, wherein the alarm device is positioned at a loadingdock.
 8. The system of claim 5, wherein the alarm device is positionedon a worker.
 9. The system of claim 5, wherein the vehicle is a firstvehicle, and further comprising a second vehicle, the alarm device beingpositioned at the second vehicle.
 10. The system of claim 9, wherein thealarm device is a first alarm device, and the system further comprises:a third generator similar to the first generator, and a second alarmdevice similar to the first alarm device; the third generator beingpositioned at the second vehicle, and the second alarm device beingpositioned at the first vehicle.
 11. The system of claim 10, furthercomprising: a first circuit connecting the first generator and thesecond alarm device, wherein the circuit disables the second alarmdevice during the time the first generator generates the magnetic field.12. The system of claim 10, wherein the second alarm device generates analarm when it senses the magnetic field of the third generator.
 13. Thesystem of claim 1, feather comprising; a plurality of vehicles;plurality of first and second field generators, each similar to thefirst and second field generators, respectively; a plurality of alarmdevices, each similar to the alarm device; each of said vehicles havinga corresponding one of the first and second field generators.
 14. Thesystem of claim 1, wherein said generator senses for a response signalfrom said alarm device after a predetermined time delay.
 15. Thecollision avoidances system of claim 1, wherein said generator is turnedon and off to generate pulses of said magnetic field.
 16. The systemclaim 1, wherein said magnetic field operates at a low frequency ofapproximately 73 kHz.
 17. The system of claim 1, further comprisingmultiple generators that generate oscillating magnetic fields in arandom manner within selected periods of time.
 18. The system of claim9, wherein said multiple generators generate oscillating magnetic fieldssequentially and not at the same time.
 19. The system of claim 1,wherein a size of the magnetic field is modified by changing a width ofa waveform input to a tuned circuit of the generator.
 20. A system,comprising: a vehicle; a first field generator that generates a firstmagnetic field; a second field generator that generates a secondmagnetic field; and an alarm device configured to detect an identity ofthe respective first and second magnetic fields based on a coding of thefirst and second magnetic fields.
 21. The system of claim 20, whereinthe coding is a time division coding.